[PATCH] HID: core: detect and skip invalid inputs to snto32()

[PATCH] HID: core: detect and skip invalid inputs to snto32()

[Randy Dunlap]

Prevent invalid (0, 0) inputs to hid-core's snto32() function.

Maybe it is just the dummy device here that is causing this, but
there are hundreds of calls to snto32(0, 0). Having n (bits count)
of 0 is causing the current UBSAN trap with a shift value of
0xffffffff (-1, or n - 1 in this function).

Either of the value to shift being 0 or the bits count being 0 can be
handled by just returning 0 to the caller, avoiding the following
complex shift + OR operations:

	return value & (1 << (n - 1)) ? value | (~0U << n) : value;

Fixes: dde5845a529f ("[PATCH] Generic HID layer - code split")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: syzbot+1e911ad71dd4ea72e04a@syzkaller.appspotmail.com
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
---
 drivers/hid/hid-core.c |    3 +++
 1 file changed, 3 insertions(+)

--- lnx-510.orig/drivers/hid/hid-core.c
+++ lnx-510/drivers/hid/hid-core.c
@@ -1307,6 +1307,9 @@ EXPORT_SYMBOL_GPL(hid_open_report);
 
 static s32 snto32(__u32 value, unsigned n)
 {
+	if (!value || !n)
+		return 0;
+
 	switch (n) {
 	case 8:  return ((__s8)value);
 	case 16: return ((__s16)value);

Re: [PATCH] HID: core: detect and skip invalid inputs to snto32()

[Jiri Kosina]

On Wed, 16 Dec 2020, Randy Dunlap wrote:

> Prevent invalid (0, 0) inputs to hid-core's snto32() function.
> 
> Maybe it is just the dummy device here that is causing this, but
> there are hundreds of calls to snto32(0, 0). Having n (bits count)
> of 0 is causing the current UBSAN trap with a shift value of
> 0xffffffff (-1, or n - 1 in this function).
> 
> Either of the value to shift being 0 or the bits count being 0 can be
> handled by just returning 0 to the caller, avoiding the following
> complex shift + OR operations:
> 
> 	return value & (1 << (n - 1)) ? value | (~0U << n) : value;
> 
> Fixes: dde5845a529f ("[PATCH] Generic HID layer - code split")
> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
> Reported-by: syzbot+1e911ad71dd4ea72e04a@syzkaller.appspotmail.com
> Cc: Jiri Kosina <jikos@kernel.org>
> Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
> Cc: linux-input@vger.kernel.org
> ---
>  drivers/hid/hid-core.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- lnx-510.orig/drivers/hid/hid-core.c
> +++ lnx-510/drivers/hid/hid-core.c
> @@ -1307,6 +1307,9 @@ EXPORT_SYMBOL_GPL(hid_open_report);
>  
>  static s32 snto32(__u32 value, unsigned n)
>  {
> +	if (!value || !n)
> +		return 0;
> +

Given the fact that this has been in the code basically since ever, we're 
probably fine, but it's good to have this fixed nevertheless. Applied 
conservatively to hid.git#for-5.12/core.

Thanks,

-- 
Jiri Kosina
SUSE Labs